using System;

using System.Linq;

using System.Collections.Generic;

​

public class Mapping

{

    public char UserId { get; set; }

    public char PermissionId { get; set; }

}

​

public class Row

{

    public List<char> UserIds { get; set; }

    public List<char> PermissionIds { get; set; }

    public override string ToString() => $"{string.Join(", ", UserIds)}  |  {string.Join(", ", PermissionIds)}";

}

public class Program

{

    public static void Main(string[] args)

    {

        HashSet<Mapping> mappings = new()

        {

            new() { UserId = 'A', PermissionId = 'X' },

            new() { UserId = 'A', PermissionId = 'Y' },

            new() { UserId = 'B', PermissionId = 'X' },

            new() { UserId = 'B', PermissionId = 'Y' },

            new() { UserId = 'B', PermissionId = 'Z' }

        };

        List<Row> rows = mappings

            .GroupBy(m => m.PermissionId)

            .Select(gr => (

                PermissionId: gr.Key,

                UserIds: gr.Select(m => m.UserId).ToArray() ))

            .GroupBy(permissionForUsers => new string(permissionForUsers.UserIds),

                ( userIds, permissionsForUsers ) => new Row {

                    UserIds = userIds.ToList(),

                    PermissionIds = permissionsForUsers.Select(m => m.PermissionId).ToList() })

            .ToList();

        Console.WriteLine(string.Join(Environment.NewLine, rows));

    }

}

​